As several of us posted from the start, the safe way is to use the OE hitch. Even if you don't want the OE hitch, the safest way to wire it is with the OE wiring harness, and I gave you a link above to buy it separately. It is the only one that will plug in to the vehicle.

Some owners have had no trouble with aftermarket wire taps, others have had similar experiences to yours. You need to decide if you are going to drive without brake lights (!!), get your mechanic to finish the job properly (if he is capable?), get someone else to try to rectify his work ($), or go back and get the OE wiring harness ($).

I think you should get the OE wiring harness, myself.

Only you can decide if your trust in your mechanic and the solution he installed is broken to the point that you want to replace the hitch as well.

I refer you to the signature line that Weasel famously uses on here: "If you think professional is expensive, just try paying for amateur"

Good luck.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue